1. Field of the Invention
The present invention relates to techniques for performing formation evaluation of a subterranean formation by a down hole tool positioned in a well bore penetrating the subterranean formation. More particularly, but not by way of limitation, the present invention relates to techniques for determining the viscosity of formation fluid drawn into and/or evaluated by the down hole tool while the down hole tool is positioned within the well bore.
2. Background of the Related Art
Well bores are drilled to locate and produce hydrocarbons. A down hole drilling tool with a bit at an end thereof is advanced into the ground to form a well bore. As the drilling tool is advanced, a drilling mud is pumped through the drilling tool and out the drill bit to cool the drilling tool and carry away cuttings. The drilling mud additionally forms a mud cake that lines the well bore.
During the drilling operation, it is desirable to perform various evaluations of the formations penetrated by the well bore. In some cases, the drilling tool may be removed and a wireline tool may be deployed into the well bore to test and/or sample the formation. In other cases, the drilling tool may be provided with devices to test and/or sample the surrounding formation and the drilling tool may be used to perform the testing or sampling. These samples or tests may be used, for example, to locate valuable hydrocarbons.
Formation evaluation often requires that fluid from the formation be drawn into the down hole tool for testing and/or sampling. Various devices, such as probes, are extended from the down hole tool to establish fluid communication with the formation surrounding the well bore and to draw fluid into the down hole tool. A typical probe is a circular element extended from the down hole tool and positioned against the sidewall of the well bore. A rubber packer at the end of the probe is used to create a seal with the wall of the well bore. Another device used to form a seal with the well bore is referred to as a dual packer. With a dual packer, two elastomeric rings expand radially about the tool to isolate a portion of the well bore there between. The rings form a seal with the well bore wall and permit fluid to be drawn into the isolated portion of the well bore and into an inlet in the down hole tool.
The mud cake lining the well bore is often useful in assisting the probe and/or dual packers in making the seal with the well bore wall. Once the seal is made, fluid from the formation is drawn into the down hole tool through an inlet by lowering the pressure in the down hole tool. Examples of probes and/or packers used in down hole tools are described in U.S. Pat. Nos. 6,301,959; 4,860,581; 4,936,139; 6,585,045; 6,609,568 and 6,719,049 and U.S. Patent Application No. 2004/0000433.
Formation evaluation is typically performed on fluids drawn into the down hole tool. Techniques currently exist for performing various measurements, pretests and/or sample collection of fluids that enter the down hole tool. Fluid passing through the down hole tool may be tested to determine various down hole parameters or properties. The thermophysical properties of hydrocarbon reservoir fluids, such as viscosity, density and phase behavior of the fluid at reservoir conditions, may be used to evaluate potential reserves, determine flow in porous media and design completion, separation, treating, and metering systems, among others.
Various techniques have been developed for determining viscosity of fluids. For example, viscometers having a bob suspended between fixation points for a torsion wire have also been proposed as described, for example, in U.S. Pat. Nos. 5,763,766 and 6,070,457. Viscometers have also been formed from vibrating objects. One such viscometer has been used in down hole applications for measuring the viscosity, density and dielectric constant of formation fluid or filtrate in a hydrocarbon producing well have been proposed. For example, International Publication Number WO 02/093126 discloses a tuning fork resonator within a pipe to provide real-time direct measurements and estimates of the viscosity, density and dielectric constant of formation fluid or filtrate within the hydrocarbon producing well. Another, having a wire clamped between two posts has been used in a laboratory environment as describe, for example in The Viscosity of Pressurized He above Tλ, Physica 76 (1974) 177-180; Vibrating Wire Viscometer, The Review of Scientific Instruments Vol. 35, No. 10 (October 1964) pgs. 1345-1348.
Despite the existence of techniques for measuring viscosity, there remains a need to provide accurate viscosity measurements down hole, and preferably without regard to the position of a sensor down hole relative to the gravitational field. It is desirable that such a system be capable of providing checks for precision and/or accuracy. It is further desirable that such a system be provided with a simple configuration adapted for use in a harsh well bore environment.